# Thermal Engineering Exam: Quiz!

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Sagar Patel
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Questions: 50 | Attempts: 284

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• 1.

### What state the first law of Thermodynamics?

• A.

Conservation of Energy

• B.

Conservation of Mass

• C.

Conservation of Momentum

• D.

Conservation of Charge

A. Conservation of Energy
Explanation
The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted from one form to another. This principle is known as the conservation of energy. It means that the total amount of energy in a closed system remains constant. This law is fundamental in understanding how energy behaves in various systems and is applicable to a wide range of fields, including physics, chemistry, and engineering.

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• 2.

### What states the second law of Thermodynamics?

• A.

All the heat entering in a system cannot be transformed to work

• B.

Entropy is always positive

• C.

Conservation of Charge

• D.

Force equilibrium

A. All the heat entering in a system cannot be transformed to work
Explanation
The second law of thermodynamics states that all the heat entering a system cannot be completely transformed into work. This means that there will always be some heat that is lost or wasted in the process. This law is based on the principle of entropy, which states that the entropy of an isolated system always increases over time. The conservation of charge and force equilibrium are not related to the second law of thermodynamics.

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• 3.

### What describes the Navier Stokes equations?

• A.

Conduction heat transfer behavior

• B.

Fluid mechanics behavior

• C.

Combustion inside a piston-cylinder assembly

• D.

Rotary Machinery

B. Fluid mechanics behavior
Explanation
The Navier-Stokes equations describe the behavior of fluid mechanics, including the motion and flow of fluids. These equations are fundamental in understanding and predicting the behavior of fluids, such as the movement of air or water. They take into account factors such as viscosity, density, and pressure gradients to model the complex behavior of fluids. This makes the answer "fluid mechanics behavior" the most appropriate choice for describing the Navier-Stokes equations.

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• 4.

### The compression ratio of an SI engine is in the range of:

• A.

4 to 6

• B.

6 to 10

• C.

6 to 8

• D.

10 to 14

B. 6 to 10
Explanation
In case of SI engine the ignition occur due to spark so compression ratio relatively low about 6 to 10

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• 5.

### The ignition quality of diesel fuel is indicated by:

• A.

Octane number

• B.

Cetane number

• C.

Flash point

• D.

Fire point

B. Cetane number
Explanation
The cetane number indicates the ignition quality of diesel fuel. It measures the delay between fuel injection and ignition, with a higher cetane number indicating a shorter delay and better ignition quality. This number is important because it affects the efficiency, combustion, and emissions of diesel engines. Octane number is used to measure the ignition quality of gasoline, while flash point and fire point are indicators of the temperature at which a fuel can ignite.

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• 6.

### When air is adiabatically saturated, the temperature attained is:

• A.

Dew point temperature

• B.

Wet bulb temperature

• C.

Dry bulb temperature

• D.

Triple point temperature

B. Wet bulb temperature
Explanation
In adiabatic process wet bulb temp remain constant

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• 7.

### A streamline and an equipotential line in a flow field.

• A.

Are parallel to each other

• B.

Are perpendicular to each other

• C.

Intersect at an acute angle

• D.

Are identical

B. Are perpendicular to each other
Explanation
A streamline represents the path followed by a fluid particle in a flow field, while an equipotential line represents a line where the potential remains constant. Since the streamline represents the direction of flow, it is perpendicular to the equipotential line, which represents a constant potential. Therefore, the correct answer is that the streamline and the equipotential line are perpendicular to each other.

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• 8.

### Which of the following is not a scalar quantity?

• A.

Force

• B.

Speed

• C.

Velocity

• D.

Acceleration

B. Speed
Explanation
Speed is not a scalar quantity because it only describes the magnitude of an object's motion without considering its direction. Scalar quantities, such as temperature or mass, can be fully described by their magnitude alone. However, speed does not provide information about the direction in which an object is moving, making it a vector quantity. On the other hand, force, velocity, and acceleration are all vector quantities as they have both magnitude and direction associated with them.

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• 9.

### In ideal machines, mechanical advantage is __________ velocity ratio.

• A.

A.equal to

• B.

B.less than

• C.

C.greater than

• D.

D.None of the above

A. A.equal to
Explanation
In ideal machines, mechanical advantage is equal to the velocity ratio. This means that the amount of force gained from using a machine is equal to the increase in speed or distance achieved. In other words, the mechanical advantage is the ratio of the output force to the input force, and it is also equal to the ratio of the output distance to the input distance. Therefore, the mechanical advantage is directly proportional to the velocity ratio in ideal machines.

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• 10.

### The units of kinematic viscosity are

• A.

Kg/m2-sec

• B.

Kg sec/m2

• C.

M/kg sec

• D.

M2/sec.

• E.

All of the Above

D. M2/sec.
Explanation
The correct answer is m2/sec. Kinematic viscosity is defined as the ratio of dynamic viscosity to density. It measures the resistance of a fluid to flow under the influence of gravity. The units of dynamic viscosity are kg/mÂ·sec, and the units of density are kg/m3. When we divide kg/mÂ·sec by kg/m3, the kg units cancel out, leaving us with m2/sec. Therefore, m2/sec is the correct unit for kinematic viscosity.

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• 11.

### Density in terms of viscosity is

• A.

Kinematic viscosity/Dynamic viscosity

• B.

Kinematic viscosity x dynamic viscosity

• C.

Dynamic viscosity/Kinematic viscosity

• D.

Any of the above

• E.

All of the Above

B. Kinematic viscosity x dynamic viscosity
Explanation
The correct answer is "Kinematic viscosity x dynamic viscosity". Density in terms of viscosity is determined by multiplying the kinematic viscosity and the dynamic viscosity. Kinematic viscosity is the ratio of dynamic viscosity to density, while dynamic viscosity is a measure of a fluid's resistance to flow. Therefore, multiplying these two values together gives the density in terms of viscosity.

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• 12.

### An object having 20 kg mass weighs 19.60 kg on a spring type balance. The value of 'g' in m/sec2 for the place is

• A.

1000

• B.

9.8

• C.

9.9

• D.

10.2

• E.

10.1

C. 9.9
Explanation
The weight of an object is given by the formula W = mg, where W is the weight, m is the mass, and g is the acceleration due to gravity. In this case, the object weighs 19.60 kg on the spring balance. Therefore, 19.60 = 20g. Solving for g, we get g = 19.60/20 = 0.98 m/s^2. Since the question asks for the value of 'g' in m/s^2, the correct answer is 9.8 m/s^2.

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• 13.

### A pilot tube is used for the measurement of

• A.

Pressure

• B.

Velocity

• C.

Viscosity

• D.

Surface tension

• E.

Flow rate

B. Velocity
Explanation
A pilot tube is used for the measurement of velocity. A pilot tube is a device that is inserted into a fluid flow to measure the velocity of the fluid. It works based on the principle of Bernoulli's equation, where the pressure difference between the static and dynamic pressure is used to determine the velocity of the fluid. By measuring the pressure difference using a pilot tube, the velocity of the fluid can be accurately determined.

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• 14.

### A hydrometer is used to determine

• A.

Relative humidity

• B.

Buoyancy force

• C.

Specific gravity of liquids

• D.

Viscosity of liquids

• E.

Surface tension

C. Specific gravity of liquids
Explanation
A hydrometer is a device used to measure the specific gravity of liquids. Specific gravity is the ratio of the density of a substance to the density of a reference substance (usually water). By measuring the specific gravity of a liquid, the hydrometer can determine the density of the liquid relative to water. This information is useful in various industries, such as brewing, winemaking, and chemical engineering, where specific gravity is an important parameter in determining the quality and composition of liquids.

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• 15.

### If the particles of a fluid attain such velocities that vary from point to point in magnitude and direction as well as from instant to instant, the flow is said to be

• A.

Uniform flow

• B.

• C.

Turbulent flow

• D.

Laminar flow

• E.

None of the above

C. Turbulent flow
Explanation
Turbulent flow is the correct answer because it is characterized by fluid particles moving with varying velocities that change in magnitude and direction from point to point as well as from instant to instant. In turbulent flow, the fluid moves in an irregular and chaotic manner, with eddies and swirls forming within the flow. This is in contrast to laminar flow, where the fluid moves smoothly and in parallel layers, and steady flow, where the velocity of the fluid remains constant over time.

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• 16.

### The equation of continuity of flow is based on the principle of conservation of

• A.

Momentum

• B.

Mass

• C.

Energy

• D.

All above

• E.

None of the above

B. Mass
Explanation
The equation of continuity of flow is based on the principle of conservation of mass. This principle states that mass cannot be created or destroyed, only transferred or transformed. The equation of continuity relates the velocity and cross-sectional area of a fluid flow to ensure that mass is conserved throughout the flow. By maintaining a constant mass flow rate, the equation of continuity helps in understanding and predicting fluid behavior in various systems, such as pipes, channels, and nozzles.

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• 17.

### Sensible heat is

• A.

That heat which causes change of state

• B.

That heat which produces a rise of temperature

• C.

That heat which produces a rise of temperature

• D.

That can only be sensed by a thermometer

• E.

None of the above

C. That heat which produces a rise of temperature
Explanation
Sensible heat refers to the heat transfer that results in a change in temperature of a substance without causing a change in its state. It is the heat that can be felt or sensed by a thermometer. This type of heat does not cause a change in the state of matter, such as melting or boiling, but it does cause an increase in temperature. Therefore, the correct answer is "That heat which produces a rise of temperature."

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• 18.

### Specific heat is

• A.

The amount of heat required to raise the temperature of a unit quantity of substance through 1?C

• B.

The amount of heat requred to change the state or condition under which a substance exists without changing its temperature

• C.

The ratio of quantity of heat required to raise the temperature of a given weight of any substance through 1?C to the quantity of heat required to raise the temperature of same weight of water through 1?C

• D.

The heat sensed by a mercury thermometer

• E.

None of the above

A. The amount of heat required to raise the temperature of a unit quantity of substance through 1?C
Explanation
Specific heat is defined as the amount of heat energy required to raise the temperature of a unit quantity (usually 1 gram or 1 kilogram) of a substance by 1 degree Celsius. It is a property that is unique to each substance and helps determine how much heat energy is required to change the temperature of that substance. The specific heat of a substance is often used in calculations involving heat transfer and thermal energy.

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• 19.

### Latent heat is

• A.

Heat cannot be detected

• B.

Heat that does not follow first law of thermodynamics

• C.

Heat required to change the substance from solid to gaseous state

• D.

Heat required to change the state of substance from liquid state to solid state

• E.

Heat required to change the state or condition under which the substance exists

E. Heat required to change the state or condition under which the substance exists
Explanation
Latent heat refers to the heat required to change the state or condition under which a substance exists. This means that when a substance undergoes a phase change, such as from solid to gaseous state or from liquid to solid state, it requires a certain amount of heat energy to make this transition. This heat energy is called latent heat because it is not detected by a change in temperature, but rather by a change in the substance's state or condition.

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• 20.

### The equilibrium conditions between supersaurated vapour and a cooled liquid surface are expressed in

• A.

Calender's equation

• B.

Dalton's equation

• C.

Kelvin Helmhotz equation

• D.

Napier's equation

• E.

Clapcyron's equation

C. Kelvin Helmhotz equation
Explanation
The Kelvin-Helmholtz equation describes the equilibrium conditions between supersaturated vapor and a cooled liquid surface. It is used to determine the rate of evaporation or condensation at the interface between the vapor and liquid phases. This equation takes into account factors such as temperature, pressure, and the properties of the liquid and vapor. It is commonly used in fields such as atmospheric science, chemistry, and engineering to understand and model phase changes and the behavior of vapor-liquid systems.

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• 21.

### An optical pyrometer works on the principle of

• A.

Expansion of fluids

• B.

Change of electrical resistance

• C.

Thermo-electric effect

• D.

• E.

Explanation
An optical pyrometer works on the principle of mono-chromatic radiation. This means that it measures temperature by detecting and analyzing the intensity of a specific wavelength of light emitted by the object being measured. By focusing on a single wavelength, the pyrometer can accurately determine the temperature based on the relationship between temperature and the intensity of that specific wavelength. This principle allows for precise temperature measurements in various industrial and scientific applications.

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• 22.

### Intercooling in multistage compressors is done

• A.

To cool the air at delivery

• B.

To enable use of small cylinders and eliminate problems of unbalanced forces

• C.

To minimise the work of compression

• D.

To supply-air at two different pressures for use

• E.

To cool air during compression

C. To minimise the work of compression
Explanation
Intercooling in multistage compressors is done to minimize the work of compression. Intercooling involves cooling the air between stages of compression, which reduces its temperature and volume. This allows for more efficient compression in subsequent stages, as cooler air is easier to compress. By minimizing the work required for compression, intercooling helps to improve the overall efficiency and performance of the compressor.

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• 23.

• A.

No heat leaves or enters the system

• B.

Maximum work is done

• C.

Specific heat remains constant

• D.

Temperature remains constant

• E.

None of the above

A. No heat leaves or enters the system
Explanation
During adiabatic compression, no heat leaves or enters the system. This is because adiabatic compression is a process in which the system is isolated from its surroundings, meaning there is no exchange of heat with the surroundings. As a result, the energy transfer in the form of heat is zero. Adiabatic compression is characterized by an increase in pressure and a decrease in volume, which leads to an increase in the internal energy of the system. Therefore, the correct answer is that no heat leaves or enters the system during adiabatic compression.

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• 24.

### Stoichiometric ratio is

• A.

Chemically correct air-fuel ratio by volume

• B.

Chemically correct air-fuel ratio by weight

• C.

Theoretical mixture of air for complete combustion

• D.

Actual ratio of air to fuel for maximum efficiency

• E.

None of the above

B. Chemically correct air-fuel ratio by weight
Explanation
The stoichiometric ratio refers to the chemically correct air-fuel ratio by weight. This means that the ratio of air to fuel is precisely balanced in order to achieve complete combustion. It is the ideal ratio at which all of the fuel is burned with just enough oxygen from the air, resulting in the most efficient combustion process. This ratio is important in determining the amount of air and fuel needed for a particular reaction, and it varies depending on the specific fuel being used.

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• 25.

### A rich mixture will have an air-fuel ratio

• A.

Less than chemically correct

• B.

More than chemically correct

• C.

Chemically correct

• D.

Any of the above

• E.

None of the above

A. Less than chemically correct
Explanation
A rich mixture refers to an air-fuel mixture that contains more fuel than the stoichiometric ratio required for complete combustion. This means that there is an excess of fuel compared to the amount of air present. As a result, the air-fuel ratio of a rich mixture is less than the chemically correct ratio. Therefore, the correct answer is "Less than chemically correct".

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• 26.

### This purpose of supercharging an engine is

• A.

To increase the power output of engine

• B.

To reduce specific fuel consumption

• C.

To reduce the noise of engine

• D.

To improve cooling of cylinders

• E.

None of the above

A. To increase the power output of engine
Explanation
Supercharging an engine involves forcing more air into the combustion chamber, which allows for more fuel to be burned and thus increases the power output of the engine. By increasing the amount of air and fuel mixture, the engine can generate more power and produce greater torque. This is particularly useful in high-performance vehicles or when additional power is needed for tasks such as towing or accelerating quickly. Therefore, the purpose of supercharging an engine is to increase its power output.

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• 27.

### In an engine radiator, heat is transferred by

• A.

Conduction

• B.

Convection

• C.

• D.

Conduction and convection

• E.

None of the above

D. Conduction and convection
Explanation
In an engine radiator, heat is transferred by conduction and convection. Conduction refers to the transfer of heat through direct contact between particles or objects. In the radiator, heat is conducted from the hot engine coolant to the metal fins of the radiator. Convection, on the other hand, involves the transfer of heat through the movement of fluid or air. In the radiator, the hot coolant flows through the tubes, and as air passes over the fins, heat is transferred from the coolant to the air through convection. Therefore, both conduction and convection play a role in heat transfer in an engine radiator.

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• 28.

### A mixture containing 60% cetane and 40% Iso-octane will have

• A.

Cetane number 60

• B.

Cetane number 40

• C.

Octane number 60

• D.

Octane number 40

• E.

None of the above

E. None of the above
• 29.

### A starting motor should have capacity to generate power equal to

• A.

IHP

• B.

BHP

• C.

IHP-BHP

• D.

Friction horse power

• E.

Friction horse power at cranking r.p.m

E. Friction horse power at cranking r.p.m
Explanation
The starting motor should have the capacity to generate power equal to the friction horse power at cranking r.p.m. This is because the friction horse power at cranking r.p.m represents the power required to overcome the friction in the engine during the starting process. Therefore, the starting motor needs to be able to generate enough power to overcome this friction and start the engine effectively.

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• 30.

### The bottom ring on the piston is known as

• A.

Oil ring

• B.

Scrapper ring

• C.

Compression ring

• D.

Groove ring

• E.

Any of the above

A. Oil ring
Explanation
The bottom ring on the piston is known as the oil ring. The oil ring is responsible for preventing oil from entering the combustion chamber and also helps to lubricate the cylinder walls. It consists of multiple rings that are designed to scrape excess oil off the cylinder walls and return it back to the oil pan. This helps to maintain proper lubrication and prevent oil consumption in the engine.

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• 31.

### The pour point of diesel fuel oil must be

• A.

More than the coldest atmospheric temperature at which oil is to be pumped

• B.

Less than the coldest atmospheric temperature of which oil is to be pumped

• C.

More than 100?C

• D.

More than 0?C

• E.

Less than 0?C

B. Less than the coldest atmospheric temperature of which oil is to be pumped
Explanation
The pour point of diesel fuel oil must be less than the coldest atmospheric temperature at which oil is to be pumped. This is because the pour point is the lowest temperature at which the oil will flow or pour. If the pour point is higher than the coldest temperature, the oil will become too viscous and thick, making it difficult to pump and flow through the system. Therefore, it is important for the pour point to be lower than the coldest temperature to ensure smooth pumping and operation.

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• 32.

### A jet engine has

• A.

Propeller in front

• B.

Propeller in back

• C.

No propeller

• D.

Propeller on the top

• E.

None of the above

C. No propeller
Explanation
A jet engine does not have a propeller. Instead, it uses a turbine to compress incoming air and mix it with fuel, creating combustion and generating thrust. The thrust propels the aircraft forward, without the need for a traditional propeller.

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• 33.

### All temperatures above the freezing point of gold are usually determined by

• A.

Thermo couple

• B.

Gas thermometer

• C.

Optical pyrometer

• D.

Resistance bridge

• E.

Any of the above

C. Optical pyrometer
Explanation
An optical pyrometer is used to measure high temperatures above the freezing point of gold. It works by measuring the intensity of the light emitted by the object being measured, and then using this information to calculate the temperature. This method is commonly used for measuring temperatures in industrial processes, such as in metalworking or glass manufacturing, where other types of thermometers may not be suitable or accurate enough. Therefore, an optical pyrometer is a valid option for determining temperatures above the freezing point of gold.

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• 34.

### The ratio of the actual vapour pressure to the pressure of saturated vapour at the prevailing dry bulb temperature is known as

• A.

Specific humidity

• B.

Vapour humidity

• C.

Relative humidity

• D.

Humidity ratio

• E.

Humidity

C. Relative humidity
Explanation
Relative humidity is the ratio of the actual vapour pressure to the pressure of saturated vapour at the prevailing dry bulb temperature. It is a measure of the amount of moisture present in the air compared to the maximum amount it can hold at that temperature. Relative humidity is expressed as a percentage and is commonly used to describe the comfort level of the air and the likelihood of precipitation.

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• 35.

### The phenomenon of boiling the milk in an open container when milk spills over the vessel is termed as

• A.

Film boiling

• B.

Pool boiling

• C.

Nuclear boiling

• D.

Sub-cooled boiling

• E.

Interface evaporation

A. Film boiling
Explanation
Film boiling is the correct answer because it refers to the phenomenon of boiling that occurs when a liquid comes into direct contact with a surface that is much hotter than its boiling point. In this case, when milk spills over the vessel and comes into contact with the hot surface of the stove, it rapidly forms a layer of vapor bubbles that insulate the liquid from further contact with the surface. This results in a violent boiling process with large bubbles and a characteristic roaring sound.

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• 36.

### A black body absorbs all radiations. As a result of absorption of these radiations

• A.

A black body shines

• B.

The temperature of black body rises

• C.

Black body radiates energy to other bodies

• D.

Black body becomes good conductor of heat

• E.

All of the Above

A. A black body shines
Explanation
A black body shines because it absorbs all radiations and then radiates energy to other bodies. This process of absorption and radiation causes the black body to emit visible light, making it appear to shine. Additionally, the absorption of radiations also increases the temperature of the black body, which further contributes to its shining appearance. Therefore, all of the given options are correct explanations for why a black body shines.

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• 37.

### Baffles are provided in heat exchangers to

• A.

Remove dirt

• B.

Increase heat transfer rate

• C.

Provide better mechanical strength

• D.

Reduce heat transfer rate

• E.

Reduce vibrations

B. Increase heat transfer rate
Explanation
Baffles are provided in heat exchangers to increase heat transfer rate. Baffles help in directing the flow of the fluid, creating turbulence, and increasing the surface area available for heat transfer. This enhances the efficiency of heat exchange between the hot and cold fluids. By promoting better mixing and reducing the formation of stagnant zones, baffles improve the overall heat transfer rate in the heat exchanger.

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• 38.

### Radiation heats transfer between two surfaces can be reduced by

• A.

Polishing the surface

• B.

Bringing the surfaces closer together

• C.

Preheating the surfaces closer together

• D.

Introducing radiation shield between the surfaces

• E.

All of the Above

D. Introducing radiation shield between the surfaces
Explanation
Introducing a radiation shield between the surfaces can reduce radiation heat transfer because the shield acts as a barrier, blocking the direct transfer of heat through radiation. This shield absorbs and reflects the radiation, preventing it from reaching the other surface. Polishing the surface can also reduce radiation heat transfer by increasing the reflectivity of the surface, causing less heat to be absorbed. Bringing the surfaces closer together can reduce radiation heat transfer by decreasing the distance over which the radiation must travel. Preheating the surfaces closer together, however, does not directly reduce radiation heat transfer. Therefore, the correct answer is introducing a radiation shield between the surfaces.

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• 39.

### The rate of heat flow between a solid wall and a fluid (in contact with a solid wall) is given by

• A.

Wein's Law

• B.

Kirchhoff's law

• C.

Newton-Rikhman's Law

• D.

None of the above

• E.

All of the Above

C. Newton-Rikhman's Law
Explanation
Newton-Rikhman's Law is the correct answer because it describes the rate of heat flow between a solid wall and a fluid in contact with the solid wall. Wein's Law, on the other hand, is used to describe the relationship between the temperature of an object and the wavelength at which it emits the most radiation. Kirchhoff's Law is used to describe the relationship between the absorption and emission of radiation by a material. Therefore, neither Wein's Law nor Kirchhoff's Law are applicable to the given scenario. "None of the above" and "All of the above" are incorrect because only Newton-Rikhman's Law is relevant.

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• 40.

### Steam traps are used to

• A.

Regulate the flow

• B.

Balance the pressure

• C.

Remove condensate

• D.

• E.

Heat steam

C. Remove condensate
Explanation
Steam traps are devices used to remove condensate from steam systems. Condensate is the liquid formed when steam cools down and loses its heat energy. If condensate is not removed from the system, it can cause damage to equipment and affect the efficiency of the steam system. Steam traps work by automatically opening to allow condensate to be drained out, and then closing to prevent steam from escaping. Therefore, the correct answer is "Remove condensate".

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• 41.

### Agitated film evaporator is suitable for concentrating

• A.

Low temperature liquids

• B.

Liquid metal

• C.

Viscous liquids

• D.

Corrosive liquids

• E.

Insulating liquids

C. Viscous liquids
Explanation
The agitated film evaporator is suitable for concentrating viscous liquids. This is because the agitated film evaporator has a design that allows for efficient heat transfer and evaporation of liquids with high viscosities. The agitation helps to create a thin film of liquid on the heating surface, increasing the surface area for evaporation and improving heat transfer. This makes it an effective choice for concentrating viscous liquids, as it can handle the challenges posed by their high viscosity and ensure efficient evaporation.

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• 42.

### Which method can be used for heating wood?

• A.

Flame heating

• B.

Resistance heating

• C.

Dielectric heating

• D.

• E.

Any of the above

C. Dielectric heating
Explanation
Dielectric heating is a method that can be used for heating wood. In dielectric heating, an alternating electric field is applied to the wood, causing the molecules within the wood to rotate and generate heat. This method is commonly used in industrial processes for drying and curing wood, as it allows for efficient and uniform heating without the need for direct contact or open flames. Therefore, dielectric heating is a suitable method for heating wood.

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• 43.

### A hollow sphere with uniform interior temperature and a small hole behaves very nearly as a

• A.

Grey body

• B.

White body

• C.

Opaque body

• D.

Transparent body

• E.

Black body

E. Black body
Explanation
A black body is an idealized object that absorbs all incident radiation and emits radiation at all wavelengths. In the given scenario, the hollow sphere with a small hole will absorb all the radiation that enters through the hole and emit radiation from its interior surface. Since the interior temperature is uniform, the radiation emitted will be independent of the wavelength, making it behave very nearly as a black body.

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• 44.

### Which of the following material has highest value of thermal diffusivity?

• A.

Concrete

• B.

Mineral wool

• C.

Wood

• D.

Steel

• E.

Explanation
Lead has the highest value of thermal diffusivity among the given materials. Thermal diffusivity is a measure of how quickly heat can be conducted through a material. Lead has a high thermal diffusivity due to its high thermal conductivity and relatively low density. This means that lead can quickly conduct heat and distribute it evenly throughout its structure. In comparison, materials like concrete, mineral wool, wood, and steel have lower thermal diffusivity values because they have lower thermal conductivity or higher density, which hinders the efficient conduction of heat.

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• 45.

### Thermal conductivity of liquids can be determined by

• A.

Guarded plate method

• B.

Searles method

• C.

Laby and Hercus method

• D.

Any of the above

• E.

None of the above

A. Guarded plate method
Explanation
The guarded plate method can be used to determine the thermal conductivity of liquids. This method involves placing a heated plate in contact with the liquid and measuring the temperature gradient across the plate. By knowing the dimensions of the plate and the heat flux applied, the thermal conductivity of the liquid can be calculated. The other methods mentioned, Searles method and Laby and Hercus method, are not commonly used for determining the thermal conductivity of liquids. Therefore, the correct answer is the guarded plate method.

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• 46.

### At all wavelengths and temperatures the monochromatic emissivity of a white body is equal to

• A.

Zero

• B.

0.1 to 0.5

• C.

0.5

• D.

0.5 to 1

• E.

Unity

A. Zero
Explanation
The correct answer is zero because a white body is a perfect reflector and does not emit any radiation. It reflects all incident radiation, resulting in an emissivity of zero at all wavelengths and temperatures.

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• 47.

### All temperatures above the freezing point of gold are usually determined by

• A.

Thermo couple

• B.

Gas thermometer

• C.

Optical pyrometer

• D.

Resistance bridge

• E.

Any of the above

C. Optical pyrometer
Explanation
An optical pyrometer is used to measure high temperatures above the freezing point of gold. It works by measuring the intensity of the thermal radiation emitted by the object being measured. This method is suitable for high temperature measurements as it does not require physical contact with the object and can provide accurate readings. Therefore, an optical pyrometer is a commonly used instrument to determine temperatures above the freezing point of gold.

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• 48.

### Plank's law of radiation is applicable to

• A.

• B.

• C.

• D.

• E.

None of the above

Explanation
Plank's law of radiation states that the intensity of radiation emitted by a black body at a given wavelength is directly proportional to the temperature and the wavelength raised to the power of 5. Monochromatic radiation refers to radiation of a single wavelength. Therefore, Plank's law is applicable to monochromatic radiation because it describes the relationship between intensity, temperature, and wavelength for a single wavelength of radiation.

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• 49.

### The value of Prandtl number for air is of the order of

• A.

1000

• B.

67

• C.

9.7

• D.

0.7

• E.

0.065

D. 0.7
Explanation
The Prandtl number is a dimensionless number that represents the ratio of momentum diffusivity to thermal diffusivity in a fluid. It is used to characterize the flow and heat transfer properties of a fluid. The Prandtl number for air is typically around 0.7, indicating that air has a relatively low thermal diffusivity compared to its momentum diffusivity.

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• 50.

### In which case the value of thermal conductivity drops with rising temperature

• A.

Methane

• B.

Benzene

• C.

Carbondioxide

• D.

Ammonia

• E.

None of the above

E. None of the above
Explanation
The value of thermal conductivity generally increases with rising temperature for most substances. However, in this case, none of the given substances (Methane, Benzene, Carbon dioxide, Ammonia) show a decrease in thermal conductivity with increasing temperature. Hence, the correct answer is "None of the above".

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• Current Version
• Mar 22, 2023
Quiz Edited by
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• Jul 20, 2019
Quiz Created by
Sagar Patel

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